A common manufacturing problem is the precise measurement of surface topography. Examples of manufactured items requiring metrology are engine parts, components for magnetic storage devices, flat-panel displays, molded and textured plastic surfaces, mechanical pump surfaces and seals, and minted coins. Efficient production requires high-speed, automated quality control in an industrial environment.
The most common measurement tools for profiling surfaces are mechanical styli, which are very slow, provide only limited information, and may damage the object surface. Automated systems commonly referred to as coordinate measurement machines are expensive, slow, and typically are used only off line rather than for continuous in-line quality control. Mechanical gauges such as micrometers and calipers have low accuracy and cannot provide profiles of surface topography. It would be very desirable therefore to provide high-speed, non-contact optical means of profiling of objects both off line and in line for precision manufacturing. Such means should accommodate a variety of object sizes, surface forms and surface textures. Such means should also be compatible with automated parts handling in an industrial environment and be insensitive to vibration.
The prior art provides several optical profiling techniques based on geometric optics. A representative example is moire fringe analysis. This technique is described in detail in Chapter 6 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). The moire method involves the projection and imaging of a ronchi ruling or like periodic structure, and is equivalent to geometric triangulation. A commercial product based on this principle is the Chek-flat, manufactured by Speedfam-Spitfire products group (Des Plaines, Ill.). Although moire is capable of profiling rough surfaces, it is generally of low accuracy when compared to mechanical stylus gauges, and in some cases does not work at all for specular surfaces. A further difficulty is that moire must be carefully calibrated for geometric errors, which are most troublesome when the object surface has large depth variations.
The prior art also provides several metrology methods using optical interferometry, which exploits the wave nature of light to map variations in surface height with high accuracy. Examples of common interferometers are given in Chapters 1 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). Most of these conventional prior-art interferometric means cannot accommodate surface features with discontinuous height variations or surface roughness that exceed one-quarter-of the wavelength of the source light, which is typically 0.63 .mu.m in commercial instruments. Discontinuous surface features larger than 0.16 .mu.m therefore result in interferometric phase ambiguities that are difficult or impossible to interpret. A further difficulty arises when the surface slope is so large that it becomes difficult to resolve or distinguish the interference fringes. Consequently, interferometers are not considered appropriate for a great number of manufacturing metrology problems.
Because of the limited range of application for conventional interferometers, the prior art provides some alternative interferometric methods and means which are compatible with rough surfaces and large variations in surface topography. One obvious approach is to increase the wavelength of the light through the use of unusual sources. An example method and apparatus is disclosed in the paper "Rough surface interferometry using a CO.sub.2 laser source," by C. R. Munnerlyn and M. Latta (Appl. Opt. 7(9) 1858-1859 (1968)). However, such methods are generally very expensive and cumbersome, since they involve specialized sources, optics and detectors. Further, even these expensive long-wavelength interferometers fail in the presence of discontinuous variations in surface topography that exceed one-quarter of the longer wavelength.
Another prior-art approach to overcoming the limited range of conventional interferometers involves the use of multiple wavelengths, as originally described by R. Rene Benoit in the paper "Application des phenomenes d'interference a des determinations metrologiques," J. de Phys. 3(7), 57-68 (1898). A sequence of measurements at two or more wavelengths provides a much larger equivalent wavelength that overcomes some of the ambiguity problems of conventional single-wavelength interferometers. A method for applying this technique to surface metrology is disclosed in U.S. Pat. No. 4,355,899, "lnterferometric distance measurement method," to T. A. Nussmeier (1982). However, these multiple wavelength techniques still do not function correctly when the surface slope is so large or the roughness so great that it becomes difficult to resolve the interference fringes. Multiple-wavelength interferometers are also extremely sensitive to vibration.
The prior art provides some alternative interferometric methods that seek to reduce the sensitivity to surface roughness and surface slope through the use of unusual measurement geometries. A representative prior-art desensitized interferometer employs an oblique angle of illumination, such as is described in the U.S. Pat. No. 4,325,637 "Phase modulation of grazing incidence interferometer" to R. C. Moore, and in the article "Oblique incidence and observation electronic speckle-pattern interferometry" by C. Joenathan, B. Franze, and H. J. Tiziani (Applied Optics 33(31), 7307-7311, (1994)). In these so-called grazing incidence interferometers, oblique angles of illumination and observation reduce the fringe density on the object surface when compared with the more common forms of interferometer. This reduced fringe density corresponds to an equivalent wavelength .LAMBDA., which may be many times longer than the actual wavelength .lambda. of the light. The larger the equivalent wavelength .LAMBDA., the greater the degree of surface roughness that can be accommodated by the instrument. However, a significant reduction in sensitivity requires a large illumination angle with respect to normal incidence. Such large angles create problems with proper illumination and imaging of the object. There may also be undesirable shadowing from surface features such as steps and channels. Additional complications arise from the need to properly balance the reference and object beams of the interferometer to compensate for variations in surface reflectivity. Further, grazing-incidence interferometers cannot accommodate surface features with discontinuous height variations that exceed one-quarter of the equivalent wavelength .LAMBDA..
Another geometric approach to generating interference patterns with a long equivalent wavelength is to divide the source light into two beams which illuminate the same portion surface at different angles of incidence. When these beams are recombined, the resulting interference pattern has a much reduced sensitivity to variations in surface topography. This reduced fringe density can also be characterized by an equivalent wavelength .LAMBDA.&gt;.lambda.; however, this method does not necessarily involve extreme angles of illumination, and has the additional benefit that the interfering beams are balanced in intensity. Instruments employing different angles of illumination and observation to achieve a long equivalent wavelength are referred to in the present disclosure as desensitized interferometers.
The prior art provides several examples of desensitized interferometers. In an article by W. Jaerisch and G. Makosch entitled "Optical contour mapping of surfaces" (Applied Optics 12(7), 1552-1557 (1973)) there is described a desensitized interferometer which employs a diffraction grating placed nearly in contact with the test surface. Illumination of the grating by a monochromatic plane wave generates several beams corresponding to different diffraction orders. These beams are reflected off of the object surface and are recombined by the grating, resulting in a fringe pattern that resembles the surface contours of the object surface. Another prior-art approach involving light beams at different angles of illumination is described in a paper "Common-path interferometer for flatness testing" by P. Jacquot, X. Colonna de Lega and P. M. Boone (SPIE 2246, Optics for productivity in manufacturing, paper 18 (1994)). This instrument works by the interaction of two diffraction orders of a holographic recording of a spherical wavefront.
Although the method of Jaerisch and Makosch and the method of Jacquot et al. have some advantages, they are not suitable for automated optical inspection, because they do not provide an adequate working distance. Both of these methods require placing a diffractive element nearly in contact with the object surface. This is because in both of these methods a single diffractive element divides the source light into beams which propagate in different directions and do not illuminate the same part of the object surface. The two beams are therefore not properly oriented for generating the desired interference effect, especially on rough surfaces. The only way to avoid this problem is to bring the object very close to the surface of the diffracting element. Typically, the working distance, defined as the distance of the object to any element of the interferometer, is less than 100 .mu.m for these systems. This is much too small a distance for most manufacturing inspection needs.
A few prior-art forms of desensitized interferometer employing two angles of illumination do not require the object to be nearly in contact with a component of the interferometer. A representative example is taught in U.S. Pat. No. 3,958,884 to F. H. Smith. Smith teaches several methods of dividing and recombining the source light using combinations of refractive and polarizing components, in such a way that the working distance is large. These methods include the use of a Jamin interferometer, a bi-refringent doublet, a bi-refringent doublet prism, or a Savart-doublet plate. A further example of a desensitized interferometer with a large working distance is provided by U.S. Pat. No. 4,498,771 "Method and means for interferometric surface topography," to G. Makosch (3:37-4:16). The apparatus disclosed by Makosch uses a birefringent crystal, such as a Wollaston prism, and a system of mirrors to direct the light beam to the object.
A disadvantage of desensitized interferometers when used with broadband or diffuse illumination is that the useful measurement depth is limited by the coherence of the light. Briefly explained, "coherence" refers here to the ability of the light source to generate interference fringes when the light is divided into two parts and recombined. Generally, the quality or contrast of the fringes declines with the difference in optical path traversed by the two beams. An incoherent source will only produce high-contrast fringes when the object is precisely positioned so that the optical path difference between the interfering beams is approximately equal to zero. The behavior of a desensitized interferometer with an incoherent source is analogous to that of a conventional white-light interferometer, such as a Mirau microscope objective, the difference being that the scale of the interference effects is enlarged. Here the term "white light" refers to any illumination that is characterized by a large spectral distribution when compared to lasers, low-pressure arc lamps and like sources of substantially monochromatic radiation. The practical consequence is that when the source is incoherent, fringes appear over only that portion of the object which falls within a small depth range along the optical axis of the interferometer. This depth range may be as small as a few times the equivalent wavelength, and is therefore too small for many kinds of large manufactured objects.
A few prior-art forms of desensitized interferometer employing two angles of illumination are insensitive to the wavelength of the source, and therefore function well in white light. One such system is disclosed in my copending United States Patent Applications entitled "Method and apparatus for profiling surfaces using diffractive optics", bearing U.S. Ser. Nos. 08/334,939 and 08/365,589, filed Nov. 7, 1994 and Dec. 28, 1994, respectively. By using combinations of two or more diffractive elements, the apparatus described in the aforementioned copending patent applications, the contents of which are specifically incorporated by reference herein their entirety, projects the plane of measurement to a convenient working distance. The preferred embodiments taught by these applications has an equivalent wavelength .LAMBDA. that is substantially independent of the source wavelength .lambda.. Therefore a variety of light sources may be used, including white-light sources. However, if the source is extended, that is, if it has a large emission area and, therefore, emits a spatially incoherent beam, the fringes appear over only that portion of the object which falls within a small depth range along the optical axis of the interferometer. This depth range may also be as small as a few times the equivalent wavelength, and is therefore also too small for many kinds of large manufactured objects.
If the object surface is rough, there is a general difficulty with all desensitized interferometers which employ beams at different angles of incidence. Good-quality fringes can only be obtained on a rough surface when the two beams impinge upon the surface at substantially the same place, even when the source light is perfectly coherent. Therefore when the object surface is rough, the measurement can only be performed over a small depth range along the optical axis of the interferometer. This characteristic of all geometrically-desensitized interferometers severely limits their usefulness.
From these observations, it may be concluded that prior-art desensitized interferometers have many advantages for certain measurement tasks that are facilitated by a long equivalent wavelength; however, they have many limitations in their ability to accommodate manufactured parts having large variations in surface topography. Further, none of the desensitized interferometers described here are capable of measuring objects having discontinuous surface features or average surface roughness exceeding one-quarter of the equivalent wavelength.
An entirely different interferometric measurement technique for surface topography measurement is based on a mechanical scanning mechanism and a conventional interferometer operating with white light. This technique is referred to in the present disclosure as scanning white-light interferometry or SWLI. A representative method for three-dimensional measurement of surface topography using SWLI is disclosed in U.S. Pat. No. 4,340,306 to N. Balasubramanian. This patent describes a white-light interferometer that includes a mechanically-scanned reference mirror, a two-dimensional detector array, and computer control. The disclosed method involves scanning either the reference mirror or the object in discrete steps, measuring the fringe contrast for each pixel at each scan position, and in this way determining for each surface point the position of maximum fringe contrast. The scan position for which the contrast is maximum is a measure of the relative height of a particular surface point. The SWLI method as taught by Balasubramanian is suitable for some specular surfaces such as optical components.
Another prior-art SWLI technique is taught in a copending United States Patent Application entitled "Method and apparatus for surface topography measurement by spatial-frequency analysis of interferograms", bearing U.S. Ser. No. 08/014,707 filed Sep. 9, 1994. This disclosed optical system for measuring the topography of an object is comprised of an interferometer with a multiple-color or white-light source, a mechanical scanning apparatus, a two-dimensional detector array, and digital signal processing means for determining surface height from interference data. Interferograms for a each of the image points in the field of view are generated simultaneously by scanning the object in a direction approximately perpendicular to the surface illuminated by the interferometer, while recording detector data in digital memory. These interferograms are then transformed into the spatial frequency domain by Fourier analysis, and the surface height for each point is obtained by examination of the complex phase as a function of spatial frequency. The final step is the creation of a complete three-dimensional image constructed from the height data and corresponding image plane coordinates.
Although the various prior-art SWLI methods and means are useful for certain kinds of objects and surfaces, they all suffer from important and fundamental disadvantages, including a small field of view, high sensitivity to variations in object reflectivity, and slow measurement time relative to most other forms of optical surface metrology, and a high sensitivity to vibration. These limitations are related to the wavelength of the light and the corresponding fringe density.
One of the most important fundamental disadvantages of prior-art SWLI is that the field of view is generally no larger than can be accommodated by standard microscope objectives. To function correctly, the detector that electronically records the interference data for a SWLI instrument must have sufficiently high resolution when compared with the interference fringe density. When the field of view of prior-art SWLI instruments is increased, the fringe density can easily become too high to resolve, especially when dealing with rough surfaces. The slope tolerance for specular surfaces decreases linearly with the field size, and the speckle effects required for rough-surface measurements are only resolvable if the numerical aperture (NA) of the objective decreases linearly as the field increases. The need to resolve the speckle pattern from rough surfaces is the most discouraging, since the amount of collected light decreases with the square of the NA. The light loss means that larger surfaces require a more powerful illuminator. Worse, the fringe contrast is now a highly variable parameter, and the quality of the measurement depends critically on the balance between the reference and object beam intensities.
Because of the difficulty in measuring large surface areas, the only available commercial instruments based on SWLI are microscopes, which accommodate a circular field of view typically less than 5 mm in diameter, such as the NewView 100 by Zygo Corporation, the RST by WYKO Corporation (Tucson, Ariz.), the MICROXAM-EX by Phase-Shift Technologies (Tucson, Ariz.), and the 512 Optical Profiler by MicroMap (Tucson, Ariz.). Therefore, in spite of the substantial need for metrology tools for manufacturing, the prior art does not provide SWLI instruments for anything other than microscopic parts.
Another fundamental disadvantage of prior-art SWLI techniques is that data acquisition is very slow. The RST manufactured by WYKO Corporation, for example, acquires data at the rate of 0.5 microns of surface depth per second. A surface with 1 mm surface features would therefore require over 30 minutes to scan. The slow speed is a consequence of the rapidly varying interference effect as a function of scan position. Accurate measurements require that these variations be recorded in detail, usually at the rate of one measurement per pixel per 75 nm of scan motion. The slow speed creates additional problems such as a high sensitivity to vibration, thermal distortions and mechanical strain during the measurement.
Another fundamental disadvantage of SWLI is its high sensitivity to vibration, which is due in part to the slow data acquisition speed, and in part to the extremely high sensitivity of the interference fringe pattern, which is easily corrupted by very small amounts of vibration. A SWLI instrument generally requires massive mounting fixtures and expensive vibration isolation. Even with these precautions, SWLI instruments are still restricted to relatively calm environments when compared to normal production environments.
As a consequence of the urgent need for high-speed, large-area metrology of manufactured parts, several prior-art attempts have been made to expand the range of application for SWLI. For example, a method taught in a paper entitled "Three-dimensional imaging by sub-Nyquist sampling of white-light interferograms," by P. de Groot and L. Deck (Opt. Lett. 18(17), 1462-1464 (1993)) involves sparse data sampling in order to improve the speed of data acquisition and therefore improve the depth range of the instrument. Nonetheless, even with sparse data sampling the SWLI approach remains fundamentally slow because of the need to accurately sample the high-density interference fringe pattern. Another prior-art attempt to increase the speed of measurement in a SWLI microscope is disclosed by L. Deck in the commonly owned U.S. Pat. No. 5,402,234 issued Mar. 28, 1995 entitled "Method and Apparatus for the Rapid Acquisition of Data in Coherence Scanning Interferometry". The system described by Deck uses a special algorithm and a data buffer to select out and store the most useful part of the interferogram for each pixel. This method substantially reduces the amount of data processing required to generate a three-dimensional image. The principles taught by Deck have been incorporated in the NewView 100 product manufactured by Zygo Corporation (Middlefield, Conn.). However, the NewView 100 still requires accurate data samples of a high-density interference fringe pattern. Consequently, the measurement speed is still very slow, and acquires data at the rate of only 2 microns of surface depth per second Another prior-art attempt to increase the useful applications SWLI is described in an article by T. Dresel, G. Haeusler and H. Venzke entitled "Three-dimensional sensing of rough surfaces by coherence radar," (Applied Optics 31(7), 919-925 (1992)). This disclosed optical system has an adjustable NA to increase the average speckle size from large, rough surfaces, and an unusual combination of two mechanical actuators, one for displacing the reference mirror over a small range, and another for scanning the object in discrete steps. Several figures in the article show graphical images of three-dimensional objects, including objects larger than 5 mm in diameter. However, the low light levels for large objects and the need to process huge amounts of data severely limit the practical value of the instrument. Also, the data acquisition procedure is exceptionally slow and unsuited to rapid optical inspection.
Thus prior art attempts to increase useful applications of SWLI can be summarized as follows: mechanical styli are useful for measuring some surface features, but are very slow, provide only limited information, and may damage the object surface; prior-art optical instruments based on geometrical optics are generally less accurate than those provided by mechanical styli and do not work on all types of surfaces; conventional interferometers are useful for very-high precision measurements of optical components, but are not suitable for most manufacturing inspection tasks; multiple-wavelength interferometers can solve some of the ambiguity problems with steps and channels on specular surfaces, but suffer from nearly the same restrictions on surface slope and roughness as single-wavelength interferometers; desensitized interferometers are advantageous for surfaces which are most easily viewed with a large equivalent wavelength, but do not work well if the surface has a large depth variation when compared to the equivalent wavelength, or when the surface has discontinuous features larger than one-quarter of the equivalent wavelength; and SWLI methods have a small field of view, are sensitive to variations in surface reflectivity, are very slow, and are extremely sensitive to vibration.
Therefore, in spite of the urgent need for non-contact optical means of profiling of objects for precision manufacturing, the prior art does not provide such means. The known methods of optical surface profiling are not compatible with an industrial environment, do not have sufficient flexibility in the size, form and texture of surfaces, and are not compatible with automated parts handling. There is accordingly an unmet need for an accurate, high-speed and flexible method and apparatus for a precise measurement of surface topography.